This invention relates to semiconductor processing equipment, and more particularly to apparatus for performing high-temperature processes upon semiconductor wafers in a thermal processing furnace.
Various types of thermal processing equipment are available commercially. During a process cycle, several boats containing silicon or other kind of wafers, supported by some kind of rod, are inserted into the furnace where the desired process is performed. Upon completion of the process cycle, the wafers are removed from the furnace and unloaded from the supporting rod. During the insertion and removal steps of the process, it is preferred that the wafers be protected from exposure to ambient air. Oxygen, water vapor and airborne particulates found in ambient air can react with the wafers and adversely affect their chemical and physical properties. One very convenient apparatus to do this has been described in U.S. Pat. Nos. 4,459,104 to Wollman and 4,543,059 to Whang et al, which describe a tubular cantilever into which the loaded wafer boats are inserted. By means of the tube, the wafers are protected against particulates, and, by flowing an inert gas through the tube, also protected from moisture and air. This is particularly important during the cantilever insertion and removal steps of a typical thermal process cycle. The tubular cantilever also reduces the amount of contaminants seen by the wafers inside the furnace tube itself since the wafer is isolated by the tubular cantilever from the furnace tube.
Further, it is important that the reaction or other processing step being performed on the wafers be carried out uniformly on all the wafers being processed, and uniformly across the surface of each wafer. Various furnace design changes have been made to assure a uniform flow of the reaction gases through the furnace tube to prevent contaminants from entering the reaction tube and adversely affecting the wafers, and to ensure efficient removal of the gases fed to the furnace.
Various diffusion furnaces have used scavenger boxes to remove these gases. These are essentially cubic in shape, with an exit port located on one side of the scavenger box. In particular, in the scavenging arrangement of the U.S. Pat. No. 4,543,059, as an example, the tubular cantilever has a provision for reactant gas exhaust through the geometric center of the end of the tube, with a curved exhaust spout used to direct the effluent stream outward and toward the vicinity of the scavenger exhaust port. There are several inherent disadvantages of this structure. First, as the gases exit the reaction chamber (i.e., the annular section between the furnace tube and the tubular cantilever) they are immediately exposed to the cubic scavenger box. The box has a very non-uniform, non-symmetrical flow pattern due to its single scavenger exhaust port. Such an abrupt change in geometry can result in radial pressure gradients at the loading/unloading end of the furnace and can tend to increase the amount of ambient air infiltration into the reaction chamber. Furthermore, the uni-directional flow pattern can stir up airborne or otherwise stationary particulates and redirect them into the reactor. The same problems, to a lesser extent, can be caused by the reactant gases which exit the inner annular section, between the tubular cantilever and the semiconductor wafers. Second, during wafer load-out, the hot wafer/cantilever load is exposed to the interior wall of the scavenger box; since oxidations are performed in the presence of corrosive, chlorine-containing compounds, stainless steel can corrode with time and leach onto the quartz tubular cantilever. Such leach products will result in high mobile ion levels as well as high defect densities associated with oxide films. Third, it is known that attaining symmetry in an oxidation reaction chamber is essential to obtain the desired high level of within-wafer and wafer-to-wafer thickness uniformities; such uniformities minimize variations in electrical characteristics of oxide films in the semiconductor devices being produced. Even though the reaction zone may appear symmetrical, an asymmetric exhaust section can seriously affect the overall symmetry of the reactor and yield wafers having a large variability in electrical and compositional characteristics.